Because they're working but not in a way that is recognized by the government data which only recognizes formal labor for a proper employer.
I would go so far as to say that the employment rate is effectively always 95%+ for every population and that the only question is whether people are working in the formal economy, in school, informally working, caring, or doing any number of other activities that make up our human society.
According to the chart in the article, the employment rate looks like 56-57% for men and just under 30% for women.
To be as generous to this hypothesis as possible, we'll say 57% for men and 29% for women.
In that case, the relevant population is 71% men.